The present invention provides an improved coring bit and method for obtaining a material core sample from the bore wall of a drilled well.
Wells are generally drilled to recover natural deposits of hydrocarbons and other desirable, naturally occurring materials trapped in geological formations in the earth""s crust. A slender well is drilled into the ground and directed to the targeted geological location from a drilling rig at the surface. In conventional xe2x80x9crotary drillingxe2x80x9d operations, the drilling rig rotates a drillstring comprised of tubular joints of steel drill pipe connected together to turn a bottom hole assembly (BHA) and a drill bit that is connected to the lower end of the drillstring. During drilling operations, a drilling fluid, commonly referred to as drilling mud, is pumped and circulated down the interior of the drillpipe, through the BHA and the drill bit, and back to the surface in the annulus.
Petroleum and other naturally occurring deposits of minerals or gas often reside in porous geologic formations deep in the Earth""s crust. These formations are targeted and slender wells are bored deep into the Earth""s crust to access and recover the reserves within the formations. Once a formation of interest is reached in a drilled well, geologists or engineers often investigate the formation and the deposits therein by obtaining and analyzing a representative sample of rock. The representative sample is generally cored from the formation using a hollow, cylindrical coring bit, and the sample obtained using this method is generally referred to as a core sample. Once the core sample has been transported to the surface, the core sample is analyzed to evaluate the reservoir storage capacity (porosity), the flow potential (permeability) of the rock that makes up the formation, the composition of the fluids that reside in the formation, and to measure irreducible water content. These estimates are used to design and implement well completion; that is, to selectively produce certain economically attractive formations from among those accessible by the well. Once a well completion plan is in place, all formations except those specifically targeted for production are isolated from the target formations, and the deposits within targeted formations are selectively produced through the well to the surface.
Several tools and methods of obtaining core samples have been used in coring. There are generally two types of coring methods and apparatus, namely rotary coring and percussion coring. Rotary coring is generally performed by forcing an open and exposed circumferential end of a hollow cylindrical coring bit against the end wall or the side wall of the bore hole and rotating the coring bit. Coring at the end wall of the bore hole and in the direction of drilling of the bore hole is generally referred to as xe2x80x9cconventionalxe2x80x9d coring. In both conventional or side wall coring, the coring tool is generally secured against the wall of the bore hole with the rotary core bit oriented towards the wall of the bore adjacent to the formation of interest. The coring bit is generally deployed in either an axial (conventional) or a radial (side wall) direction away from the coring tool and against the bore wall by an extendable shaft or other mechanical linkage. The coring tool generally simultaneously imparts rotational torque and axial force (weight on bit) to the core bit to affect cutting of a core sample. The circumferential cutting edge of the bit is usually embedded with carbide, diamonds or other hard materials with superior hardness for cutting into the rock comprising the target formation. As the core sample is cut, the cylindrical core sample is received within the hollow barrel of the coring bit as cutting progresses and the bit penetrates the formations. After the desired length of the core sample or the maximum extension of the core bit is reached, the core sample may be broken from the remaining interface or connection with the formation by slightly tilting the bit and the protruding core sample within the bit from their cored orientation.
In side wall rotary coring, the core sample is broken free from the formation and the core sample is retrieved into the coring tool through retraction of the same shaft or mechanical linkage that was used to deploy the coring bit to and against the side wall. Once the coring bit has been retracted within the coring tool, the retrieved core sample is generally ejected from the coring bit to allow use of the coring bit for obtaining subsequent samples at the same or other formations of interest. This multiple coring feature is generally unavailable with conventional coring.
The second common type of coring is percussion coring. Percussion coring uses multiple cup-shaped percussion coring bits that are propelled against the wall of the bore hole with sufficient force to cause the bits to forcefully enter the rock wall such that core samples are obtained within the open end of the percussion coring bits. These bits are generally pulled from the bore wall using flexible connections between the bit and the coring tool such as cables, wires or cords. The coring tool and the attached bits are returned to the surface, and the core samples are recovered from the percussion coring bits for analysis.
The selection of either rotary or percussion coring is generally based on several factors. For certain types of rock, percussion coring provides limited useful information because the violent impact of the bit physically fractures and damages a localized portion of the bore wall including the portion recovered as the core sample. For these types of formations, rotary coring is the preferred method of obtaining a core sample that retains its natural properties and will provide reliable geologic data. However, rotary coring with prior art coring bits may also damage certain types of core samples and thereby compromise the value of the data obtained from analysis of the core sample. Many types of unconsolidated formations comprise a relatively soft matrix containing harder rock particles dispersed within the matrix. Core samples from these unconsolidated formations may be damaged, fractured or shattered when cut or removed by rotary coring bits, not to mention percussion bits, because prior art coring bits are generally rigid with carbide or diamond xe2x80x9cteethxe2x80x9d that are incompatible with the physical properties of unconsolidated formations.
The retrieval and analysis of core samples in their undamaged condition provides valuable geologic information that drastically improves analysis and decision making on the part of the driller. What is needed is an improved coring bit and method of obtaining core samples that better cuts and preserves core samples from unconsolidated, soft or matrix formations, and that provides core samples at or near their original, undamaged condition within the formation. It is preferred that the improved coring bit and method be useful with existing coring tools.
The present invention provides a brush bit for improved cutting of core samples from unconsolidated formations, and a method of cutting a core sample using a brush instead of rigid cutting teeth. The brush bit uses a plurality of protruding stiff, flexible bristles to more delicately xe2x80x9ccutxe2x80x9d an unconsolidated rock matrix to create a protruding core sample that can be retrieved to within the coring tool. The resulting core sample is either undamaged or less damaged than by forceful displacement of dispersed rock particles within the softer formation matrix. The bristles of the brush bit may be of various lengths, gauges, spacings and firmness, and may be arranged in any pattern that facilitates cutting of the core sample. The bristles of the brush bit may be braided or twisted together, bundled or may extend separately from the base of the brush bit. The brush bit may be rotated like conventional rotary coring bits, or it may be oscillated or vibrated in a manner that causes the desired removal of formation material from around the core sample. The base of the brush bit may have internal or external grooves or channels to assist in removal of cuttings and debris or to impart a secondary reaming or boring effect to the brush bit.